This invention relates to electric transformers of the dual voltage type, and more particularly, to voltage surge or impulse protection for dual voltage distribution transformers.
Dual voltage distribution transformers are generally understood in the art as being those distribution transformers having two primary voltage ratings, typically a higher voltage rating and a lower voltage rating. One way of achieving this dual voltage rating is to provide the transformer with a plurality of primary windings which are connected in series to effect the higher voltage rating or are connected in parallel to effect the lower voltage rating. Conversion from one voltage rating to the other is effected by operating a special switch, commonly referred to as a dual-voltage switch. In a first condition, this switch connects the windings in series, and in a second condition, the switch connects the windings in parallel. A dual voltage transformer that employs this approach is illustrated in U.S. Pat. 4,621,298--McMillen, assigned to the assignee of the present invention.
The above approach to providing a dual voltage rating is satisfactory if the dual voltage ratio (i.e., the ratio of the higher voltage rating to the lower voltage rating) is less than about 4 to 1. But if this ratio appreciably exceeds this value, it becomes necessary with this approach to employ an awkward, bulky, and complex design for he dual voltage switch and the associated wiring that connects the switch with the windings. Generally speaking, the greater this ratio, with the above-described approach, the more bulky and complex becomes the design of the dual voltage switch and the associated wiring.
We use, for providing dual-voltage rating capability, an approach different from the above that allows the dual voltage switch and the associated wiring to be of a less bulky and complex design, especially when the dual voltage ratio is greater than 4 to 1.
The approach that we use involves providing the primary winding with a tap at a location intermediate its length that is kept idle when the transformer is being used at its higher voltage rating. The primary winding has a first termination at one end for connection to line voltage when the transformer is being used at its higher voltage rating and a second terminal at its opposite end for connection to ground. When the transformer is to be operated at its lower voltage rating, line voltage is connected to the tap and is disconnected from said first termination except through a then-idle portion of the primary winding that extends between the tap and the first termination.
The above-described tap may be located at any desired point along the length of the primary winding to provide the desired dual-voltage ratio. This is advantageous in terms of flexibility and simplicity; but a problem with this approach is that when the transformer is connected for its lower voltage rating, the then-idle portion of the primary winding is vulnerable to damage from surge voltages. The primary winding then has an autotransformer configuration, and an incoming surge appearing across the active portion of the primary winding will be transformed by autotransformer action to a much higher voltage surge across the then-idle portion of the winding between the tap and the first termination. In general, the higher the dual voltage ratio, the higher will be the peak to which this higher voltage surge tends to rise.